Electrolytic titration apparatus



Feb. 11, 1969 I A. R. MYERS ETAL ELECTROLYTIC TITRATION APPARATUS ofSSheet Filed Nov. 18, 1964 Anode Sensor E y C M Ma Rm Referoncc MAGNETICSTIRRER [7&6 @2653 Attorneys ELF.G'IROLY'IIC TITRATION APPARATUS FiledNov. 18, 1964 Sheet 2 of :5

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ELECTROLYTIC TITRATION APPARATUS Filed Nov/ 18, 1964 Sheet i of 5 AnodeReference i .INVENTORS A. Robert Myers B James A. McNu Iry United StatesPatent 8 Claims ABSTRACT OF THE DISCLOSURE Electrolytic titrationapparatus which has chopper means for supplying power to the generatingelectrodes only when the sensing electrode assembly is disconnected andwhich has isolation means for providing a predetermined impedancebetween the sensing electrode and the reference electrode of the sensingassembly.

This invention relates to an electrolytic titration apparatus and moreparticularly to an electrolytic titration apparatus which can beutilized for macro, micro, and sub-micro analysis.

In US. Patent No. 3,032,493, there is disclosed electrolytic titrationapparatus which can be utilized for micro-analysis. Although suchelectrolytic titration apparatus has been found to be commerciallysuccessful, there is still a need for apparatus with increasedsensitivity and particularly an improved signal to noise ratio.

In general, it is an object of the present invention to provide anelectrolytic titration apparatus which has increased sensitivity.

Another object of the invention is to provide apparatus of the abovecharacter in which an improved titration cell is utilized.

Another object of the invention is to provide apparatus of the abovecharacter in which the signal to noise ratio is substantially improved.

Another object of the invention is to provide apparatus of the abovecharacter in which improved circuitry havin novel isolation features isprovided.

Additional features and objects of the invention will appear from thefollowing description in which the preferred embodiment is set forth inconjunction with the accompanying drawings.

Referring to the drawings:

FIGURES 1A and 1B show an electrolytic titration apparatus incorporatingthe present invention in which FIGURE 1A is a top plan view of thetitration cell and FIGURE 1B is a circuit diagram of the circuitryconnected to the titration cell.

FIGURE 2 is a top elevational view of the titration cell.

FIGURE 3 is a front elevational view of the titration cell shown inFIGURE 2.

FIGURE 4 is a cross-sectional view taken along the line 44 of FIGURE 2.

FIGURE 5 is an enlarged cross-sectional view of the cathode assembly ofthe titration cell.

FIGURE 6 is an enlarged cross-sectional view of the reference assemblyof the titration cell.

FIGURE 7 is a cross-sectional view taken along the line 7-7 of FIGURE 6.

The titration cell 11 shown in the drawings and forming a part of theelectrolytic titration apparatus includes means containing anelectrolyte and defining a reaction zone 12 for carrying out thetitration process as hereinafter described. This means is shown in theform of a substantially cylindrical vessel or cell 13 formed of asuitable material such as glass. As can be seen, the ves- 3,427,238Patented Feb. 11, 1969 sel 13 is provided with a straight-sidedcylindrical portion 13a which is formed integral with a footing 14 thatserves to support the cylindrical portion 13a in a substantiallyvertical position. The vessel 13 is also provided with an enlarged orflared portion 13b, a cylindrical recessed portion 130, a straight-sidedportion 13d and an enlarged upper portion 13e.

A cap 16 is mounted in the top portion of the vessel 13 and serves toenclose the top of the vessel. The cap is provided with a dome-shapedupper portion 16a and a relatively straight-sided portion 16b. The cap16 is formed of a suitable material such as glass and the outer surfaceof the portion 16b is provided with a surface which can mate with theinner surface of the portion 13d of the vessel 13. Thus, the innersurface of the portion 13d and the outer surfaces of the portion 16b areground so that both surfaces mate with each other to provide arelatively tight fit in which the portion 13d serves as the femaleportion and the portion 16b serves as the male portion of a ground glassjoint which is formed between the cap 16 and the vessel 13.

The titration cell is formed with a gas inlet tube 18 which serves asmeans for introducing the constituent to be titrated or analyzed intothe titration cell. As can be seen from FIGURE 2, this inlet tube 18 isconnected to the lower portion of the vessel 13 and is provided with aportion 18a which extends outwardly therefrom. A portion 18b extendsupwardly to a position which is substantially above the level of theelectrolyte within the cell. The tube is provided with another portionwhich extends in -a horizontal outward direction. The tube 18 is alsoprovided with an enlarged funnel-shaped portion 18d. An integral brace19 is provided for connecting the upper portion of the tube 18 to thevessel 13 to give additional support to the tube 18. A small passage 21having a diameter of .013 of an inch is formed in the inlet tube 18 andextends from the funnel portion 18d into the reaction zone 12 in thevessel 13 as can be seen from FIGURE 2.

A pair of drain tubes or side arms 22 and 23 are also mounted on thelower portion of the vessel 13 and extend outwardly therefrom in ahorizontal direction. A stop cock 24 is provided in each of the draintubes 22 and 23. Passages 26 and 27 are provided in the drain tubes 22and 23, respectively, and extend through the length thereof and openinto the reaction zone 12 formed within the vessel 13. As can be seenfrom FIGURES 1A and 2 of the drawings, the passages 26 and 27 havevarying crosssectional areas with the passage having a relatively smalldimension as it enters the reaction zone and having a relatively largecross-sectional area at an intermediate portion of the drain tubes.

A reference electrode assembly 29 is connected to the drain tube 22 anda cathode assembly 31 is connected to the drain tube 23. The referenceelectrode assembly consists of a vertical arm 32 which is connected tothe side arm 22. The arm 32 is provided with a vertically extendingpassage 33 which connects with the enlarged portion 26b of the passage26 in the side arm 22 so that a liquid bridge is formed between the arm32 and the reaction zone 12 within the vessel 13. A reference electrode34 of a suitable metal, as hereinafter described, is removably disposedwithin the passage 33. The electrode 34 is supported by support member36. The support member 36 is provided with a ground glass surface 36awhich mates with a female ground glass portion 32b formed in the upperpart of the vertical arm 32 to form a ground glass joint. The referenceelectrode 34 is provided with a coil portion 34a and a straight portion3412 which is mounted within the lower portion of the support member 36and which is connected to a female connector 37 disposed within theupper portion of the support member 36. As can be seen particularly fromFIGURE 2, the support member 36 mates with the vertical arm 32 and thereference electrode 34 is mounted in the support member 36 in such amanner that the lower extremity of the reference electrode extends fromthe bottom of the passage 33.

Means is provided for establishing consistency of the liquid bridgewhich is formed in the passage 33 and consists of a cylindrical member39 which is disposed within the lower extremity of the passage 33 belowthe reference electrode 34. This member 39 is provided with a largenumber of uniform diameter, small capillary passages 41 which connectthe passage 33 with the passage 26 to provide a uniform impedance. Anintegral brace 42 connects the upper portion of the vertical arm 32 tothe vessel 13 to give additional rigidity to the arm 32.

The cathode electrode assembly 31 consists of a vertical arm 44 which isconnected to the side arm 23. A generating cathode 46 is mounted in theportion 27b of the passage 27 and is provided with a coil or helicalportion 46a and a straight portion 46b which extends upwardly into thearm 44 and is sealed with respect thereto. The portion 46b is connectedto a female receptacle 47 mounted in the arm 44. As can be seen, thegenerating cathode 46 is disposed in a horizontal position within thepassage 27. An integral brace '48 is provided for connecting the upperpart of the arm 44 to the vessel 13 to provide adidtional support forthe arm 44.

A generating anode 51 and a sensor or sensing element 52 are disposedwithin the vessel 13 Within the reaction zone 12. Both the generatinganode 51 and the sensing element 52 are in the form of flat rectangularplates and are formed of suitable materials as hereinafter described.Means is provided for supporting the generating anode 51 and the sensingelement 52 within the reaction vessel and consists of substantiallyparallel glass tubes 53 and 54 which are secured to the top 16 in such amanner so that they extend downwardly from the top in a substantiallyvertical direction. The generating anode 51 and the sensing element 52are connected by leads 56 to female sockets provided in the upperextremities of the tube similar to that hereinbefore described inconnection with the reference electrode and the generating cathode. Thecathode 46 and the anode 51 serve as generating electrodes whereas thereference electrode 34 and the sensing electrode 52 serve as a sensingelectrode assembly.

A funnel 58 is provided in the top cap 16 to facilitate introduction ofeletcrolyte into the reaction zone 12 within the vessel 13.

In examining the arrangement of the electrodes, it can be seen that thesensing element 52 is positioned in front of the opening 21 provided inthe gas inlet 18 and that the generating anode 51 is removed 180therefrom. The inlets to the side arms 22 and 23 are also spaced 180apart and are removed 90 from the inlet opening 21.

Means is provided for stirring the electrolyte within the reactionchamber 12 and consists of a metal cylinder 61 which is rotated withinthe reaction zone 12 by suitable means such as a magnetic stirrer 62.

The titration cell 11 with its magnetic stirrer 62 are connected tocircuitry shown in FIQURE 1B to provide a complete electrolytictitration apparatus. As shown in FIGURE 1B, this circuitry is connectedto a recorder 64 of a suitable type such as a strip chart recorder. Thecircuitry includes a number of elements including chopper A and chopperB which are pointed out during the description of operation of theapparatus.

Operation of the electrolytic titration apparatus may now be brieflydescribed as follows. Let it be assumed that the vessel 11 has beenfilled with electrolyte through the funnel 58 to a suitable level suchas the level 66 as shown in FIGURE 4. Electrolyte is then permitted topass into the side arms 22 and 23 by opening the stop cocks 24.

Now let it be assumed that the fluid stream containing the sample to betitrated is passed through the passage 21 of the inlet tube 18 throughthe capillary opening and into the electrolyte within the reaction Zone12. In the case of a gas stream, excess inert gas similar to the samplecarrier may be added to the main stream to ensure rapid and completeintroduction of the sample into the cell. This higher rate of flowreduces the possibility of the electrolyte or the solution in thereaction chamber from backing up into the inlet tube 18. Also, in thissame manner, sample hold-up on the wetted surface of the inlet tube 18is substantially reduced, if not prevented.

During the time that the sample is introduced, the magnetic stirrer 62is placed in operation to cause stirring of the electrolyte and to causethe sample to be mixed rapidly and uniformly distributed through theelectrolyte.

As the sample enters the reaction zone 12, it goes into the solutionwithin the vessel 13 to produce a change in the potential between thesensing electrode 52 and the reference electrode 34. The manner in whichthe sample goes into the solution need not be known. It may be byreaction or hydration, or a combination of both. It makes no differenceas far as the present analysis is concerned.

As can be seen, the circuitry includes principally two choppers, chopperA and chopper B, and power means in the form of an amplifier A-l. Boththe choppers and the amplifier are conventional. The choppers are,however, of the 60 cycle synchronous type. A function switch S is alsoprovided which has five different wipers S1, S2, S3, S4 and S5 engagingfive levels of contacts, with each level of contacts being numbered 1through 5. The first position for the function switch is when the wiperarms are on the #1 contact. In this position, the apparatus is in an offcondition. The second contact of each bank can be identified as the biasread position of the function switch S. In this position, the biassupplied to the wiper S2 is adjusted by adjusting the potentiometer R1.This bias is obtained from the battery B1 through the fixed resistor R2.In the #2 position of the function switch, a bias voltage is suppliedthrough a resistor R3, through the wiper S5 and through the meter M1. Bywatching the meter M1, the desired amount of bias as, for example, 250millivolts, can be readily obtained. As is well known to those skilledin the art of operating titration cells, this bias level is a functionof the silver ion concentration in the electrolyte.

After this is completed, the function switch is shifted to the #3position, which can be called the generator read position, to actuallydetermine the potential of the electrolyte within the reaction zone byconnecting the output of the amplifier A to the meter M1. This makes itpossible to check whether or not a fresh electrolyte or additionalelectrolyte is required to bring the electrolyte up to the desiredpotential.

After it has been established that the bias potential and the potentialof the electrolyte are substantially identical as, for example, 250millivolts, the function switch S4 is shifted to the fourth position,which may be called the operate position. This will automatically bringthe titration cell to the bias potential which is supplied to the wiperS2.

Now let it be assumed that the fluid stream containing the sample to betitrated is passed through the inlet tube 18, through the capillarypassage 21 into the electrolyte in the reaction zone 12. As the sampleenters the reaction zone 11, it goes into the solution to produce achange in potential. This change in potential is caused by a decrease inthe silver ion concentration which demands current flow between thegenerating cathode and anode. This current flow in the reaction zone 12is such that it causes an incerase in silver ion concentration. Thiscurrent flows for a period of time and is recorded on the recorder 64 ina manner hereinafter described.

The choppers A and B operate synchronously, that is, when contacts 1 and2, and 4 and 5 of chopper A are closed, the corresponding contacts ofchopper B will also be closed. The choppers A and B operate at a 60cycle rate.

When the chopper A and chopper B are in the positions shown in FIGURE1B, that is, with contacts 1 and 2, and 4 and 5 of each of the choppersclosed, the potential being sensed by the sensor electrode is beingmeasured by the capacitor C1 connected across the contacts 2 and 5 ofchopper A. At the same time the input and output of the amplifier A1 aregrounded through R4 and R8, respectively. Under this condition, it canbe seen that the reference sensor electrodes are completely isolatedfrom the generating anode and cathode because there is no directconnection between the sensor and reference electrodes and thegenerating anode and cathode.

When the choppers A and B are operated so that contacts 2 and 3, and 5and 6 of each of the choppers are closed, the potential which was sensedby the sensor electrode and placed on the capacitor C1 is compared withthe bias voltage supplied by the wiper S2. Any difference between thebias voltage and the voltage on the capacitor C1 is supplied through thecoupling capacitor C2 to the amplifier A-1 through closed contacts 2 and3 of chopper B. In view of the fact that choppers A and B operatesynchronously, the signal being repeatedly applied to the capacitor C2is pulsating DC which is coupled to the AC amplifier A-l which amplifiesthe same and supplies the same to the capacitor C3 and resistor R6through the closed contacts 5 and 6 to place a potential on thecathodeanode circuit in the form of a pulsating DC. Current flowsbetween the cathode and the anode to cause the generation of titrant.The amount of current which flows is stored by the capacitor C3. As iswell known to those skilled in the art, the amount of reactant generatedis directly proportional to the current generated. The charge on thecapacitor C3 is supplied to the recorder when the contacts 4 and 5 ofthe chopper B are closed through the resistor R9 and through theresistive bridge network consisting of the resistors R10, R11, R14 andR15. Generation of the titrant continues until sufficient titrant hasbeen generated to cause the system to reach a null condition, at whichtime the potential sensed by the sensing electrode is substantiallyidentical to the bias electrodes so that a signal is no longer suppliedto the amplifier A1 and no potential is supplied to the cathode.

If an excess of titrating agent is generated in error, the polarity ofthe signal at the indicating or sensing electrode is reversed to causecurrent to fiow in the opposite direction from the output of theamplifier and to reverse the normal electrolytic action until the excessof the titrating agent has been removed.

The output of the amplifier A-l in positions 2, 3 and 5 of the functionswitch S is supplied directly to the meter M1 so that the meterindicates the amplitude of the output voltage developed by theamplifier. The resistor R7 is a calibration resistor for the meter M1when it is reading the output voltage of the amplifier. The resistor R8is a range resistor and establishes a range of currents which arerecorded by the recorder 64. The resistor R4 protects the contacts ofthe chopper B. Resistor R5 serves as a load resistor for the amplifieron the generator read and standby positions of the function switch S.

The potentiometer R1 provides an adjustment for the bias voltage. Theresistor R2 establishes a full scale voltage across the resistor R1, andthe resistor R3 is provided for calibrating the meter M1.

The resistor R9 and the capacitor C2 provide an integrating or filternetwork which reduces the amplitude of small noise voltages beingdeveloped in the circuitry. The resistors R10, R11, R14, R and R16 forma bridge circuit in which a battery B1 of a suitable voltage such as1.45 volts in series with the resistor R12 establishes a predeterminedvoltageacross the bridge. The resistor R15 serves as a center controlfor the recorder. A resistor R17 in series with a marking switch makesit possible for the marking switch to unbalance the bridge to generatean upswing on the recorder pen to mark a point on the recorder.

It should be pointed out that for substances titratable with silver ion,the sensor and generator anode are silver, and the generator cathode isplatinum, and the reference is silver in saturated silver acetate. Thesupporting electrolyte is 70-85% acetic acid. For those substancestitratable with iodine, the electrodes are of noble metal and thereference electrode is platinum in saturated triiodide, the supportingelectrolyte is 0.04 to 0.05% KI and 0.4% acetic acid. Other electrodesand electrolyte systems can be used. However, the iodine and silvertitrations are the most common.

The titration cell is particularly sensitive because the sensorelectrode is placed directly adjacent the gas inlet so that a muchlarger signal output is provided for a given quantity of sample injectedinto the electrolyte. Another factor is the fact that the generatinganode 51 is positioned from a gas inlet so, therefore, for that reasonit is exposed to less turbulence caused by the bubbles formed uponintroduction of the sample into the electrolyte. This is because anybubbles which are formed normally dissipate about the time liquidcirculates around the anode 51. Greater sensitivity is also obtainedbecause the reference electrode arm 22 and the cathode arm 23 enter thereaction zone of the cell 180 apart. It can be seen that the arrangementof the electrodes is such that the electrodes are positioned verticallyso that the same strata of liquid is scanned by the electrodes. Thus,the anode is disposed at substantially the same vertical eleva tion asis the sensor.

The plug or member 39 which is provided with small passages 41 isparticularly important in that it ensures uniformity of impedance in theliquid path formed between the passage 33 and the passage portion 26a ofthe passage 26.

One particularly important feature of the present apparatus is thatthere is no interaction between the reference and sensor electrodes andthe anode and cathode generating electrodes because of the use of thesynchronous choppers. This feature makes it possible to place theelectrodes in any position in the cell and also makes it possible toplace the sensor directly over the gas inlet thereby making it possibleto obtain a larger signal for a given sample introduced into the cell.

From the foregoing, it can be seen that with the electrolytic titrationapparatus herein disclosed, an error signal is amplified, polarized andcoupled to the generating electrode circuit by another chopper. There isno servo system and, therefore, there is no dead band. The circuitryprovides a very tight control and extremely fast response. In the ofi?position, the reference and sensor electrodes are completely removedfrom the circuit by center off chopper swingers. In the standbyposition, the generating electrodes are disconnected from the circuit topermit scrubbing a sample stream. The amplifier output can then beswitched into the cell when desired to complete the titration. Thesensor electrode is isolated from the effects of the applied generatorvoltage by the phasing of the chopper contacts. A greatly improvedsignal to noise ratio is also obtained.

We claim:

1. In an electrolytic titration apparatus, an electrolytic cellcomprising means containing an electrolyte forming a reaction zone,means for introducing a fluid containing the constituent to be titratedinto the electrolyte, generating electrodes disposed in the electrolyte,a sensing electrode assembly disposed in the electrolyte, and circuitmeans connecting said generating electrodes and said sensing electrodeassembly and causing the generating electrodes to generate a titrant inaccordance with the amount of constituent introduced into theelectrolyte, said circuit means including storage means, power meanshaving an input and an output and chopper-operated means for connectingthe storage means alternately to the sensing electrode assembly and tothe input of the power means, and connecting the generating electrodesto the output of the power means in such a manner that power is suppliedto the generating electrodes from the power means only when the sensingelectrode assembly is disconnected from said storage means to therebyisolate said sensing electrode assembly from said generating electrodes,and the electric fields thereabout.

2. An apparatus as in claim 1 together with indicating means and whereinsaid chopper-operated means connects the output of the powers meansalternately to the generating electrodes and to the indicating means.

3. An apparatus as in claim 1 wherein said chopperoperated meansincludes a pair of synchronous choppers each having two sets ofcontacts.

4. An apparatus as in claim 1 wherein said sensing electrode assemblyincludes a reference electrode and a sensing electrode together withmeans for isolating the reference electrode from the reaction zone, saidmeans for isolating including a member having a plurality of relativelysmall capillary passages of a predetermined number extendingtherethrough of substantially uniform diameter, said member serving toprovide a predetermined impedance between the reference electrode andthe electrolyte in the reaction zone.

5. An apparatus as in claim 1 wherein said generating electrodes consistof a cathode and an anode and said sensing electrode assembly consistsof a sensing electrode and a reference electrode, wherein said circuitmeans includes a bias supply and indicating means and wherein saidchopper-operated means includes first and second choppers, each of saidchoppers having first and second sets of contacts, each set of contactscomprising a common contact and first and second switch contacts, meansconnecting said storage means between the common contacts of said firstand second sets of contacts of said first chopper, said first switchcontact and said first set of contacts of said first chopper beingconnected to said reference electrode and said first switch contact ofsaid second set of contacts of said first chopper being connected to thesensing electrode, means for connecting the second switch contact of thefirst set of contacts of said first chopper to the second switch contactof said first set of contacts of said second chopper, means connectingthe second switch contact of said second set of contacts of said firstchopper to the bias supply, means connecting the common contact of saidfirst set of contacts of said second chopper to the input of the powermeans, means connecting the common contact of the second set of contactsof said second chopper to the output of the power means, meansconnecting the first switch contact of the first set of contacts of thesecond chopper to ground, means connecting the first switch contact ofthe second set of contacts of the second chopper to the indicating meansand means connecting the second switch contact of the second set ofcontacts of the second chopper to the cathode electrode of thegenerating electrodes.

6. Apparatus as in claim 5 wherein said power means is an amplifier andwherein said means connecting the common contact of said first set ofcontacts of the second chopper to the input of the power means includesa capacitor connected to the input of the amplifier and wherein saidmeans connecting the common contact of the second set of contacts of thesecond chopper to the output of the power means includes a capacitorconnected to the output of the amplifier.

7. Apparatus as in claim 5 wherein said first and second choppers aresynchronously driven.

8. In an electrolytic titration apparatus, an electrolytic cellcomprising means containing electrolyte forming a reaction zone, meansfor introducing a fluid containing the constituent to be titrated intothe electrolyte, a pair of generating electrodes disposed in theelectrolyte, a sensing electrode and a reference electrode disposed inthe electrolyte, and means for isolating the reference electrode fromthe reaction zone, said means including a member having a plurality ofrelatively small capillary passages of a predetermined number extendingtherethrough of substantially uniform diameter, said member serving toprovide a predetermined impedance between the reference electrode andthe electrolyte in the reaction zone.

References Cited UNITED STATES PATENTS 2,624,701 1/ 1953 Austin 204-2,851,654 9/1958 Haddad 204-195 2,886,496 5/ 1959 Eckfeldt 2041952,936,423 5/1960 Berry 330-9 3,032,493 5/ 1962 Coulson et a1. 204--195OTHER REFERENCES Epstein et al., Analytical Chemistry, September 1947,PP. 675-677.

ROBERT K. MIHALEK, Primary Examiner.

T. TUNG, Assistant Examiner.

US. Cl. X.R.

